The invention relates to apparatus for extruding and in particular to an apparatus and method for extruding pencils from synthetic materials such as plastics.
Wood substitute materials have been known for many years and generally comprise a foamed thermoplastic such as polystyrene containing fillers for colouring and fibrous fillers to provide texture. Such materials are also known for use as sheath materials for pencils. For example U.S. Pat. No. 3,993,408 discloses pencils made with a resin-based sheath consisting essentially of a resin binder, a fibrous filler and a metallic soap. The material is extruded around a lead or marking core. The core material is a traditional ceramic. The extrusion process comprises pre-heating the core material to approximate the temperature of the molten sheath material that is coated onto the core in a crosshead die of conventional construction. The extrudate is chilled and then cut once cooled.
The main problems with using substitute materials for pencils are in providing the correct properties. Pencils must be easily sharpened and whether this is possible depends on how hard the sheath material is and also, because of the impacting torsion, on how brittle the lead is; it also depends on the adhesion between the lead and the sheath. The lead must be of constant thickness and the manufacturing process must not introduce variations in tensions or show any fractures.
GB-A-1363161 gives suitable synthetic compositions for pencil casings. The composition is a polystyrene, polyethylene and glass fibre composition together with blowing agents, say, 0.2-1.5%. The synthetic pencils have the required sharpenability together with a flexural modulus greater than 30000 Kg/cm2.
Pencils are made increasingly of synthetic materials, whereby profiles are extruded and subsequently cut to the length of a pencil. The binding material and the coating material (sheath) use various polymer materials but SAN and ABS polymers have previously been preferred.
In U.S. Pat. No. 5,531,947 a polymer binding material is melted in a lead extruder in order to manufacture a synthetic lead. In a loading extruder a mixture of additives is added to the melted polymer material, including colours, gliders and adhesive substances. A substitute lead is extruded and cooled in a first cooling range to the point where, upon entering the coating nozzle the xe2x80x98leadxe2x80x99 is no longer molten but is not completely cold. In a coating extruder the coating materials are introduced for the formation of the pencil coating or sheath. The lead extruder includes first and second loading stations a melting and transportation range and a mixing and kneading range downstream of the second loading station. First and second cooling ranges lie between the lead extruder and the coating nozzle and downstream of the coating nozzle respectively.
U.S. Pat. No. 3,936,519, discloses apparatus for extruding a lead pencil in which the lead is securely bonded to a sheath formed from foamed plastic material.
It is an object of the present invention to provide an improved extrusion process and apparatus for manufacturing pencils and the like. It is also desirable as much as possible to recycle materials such as plastics. More particularly where the xe2x80x98leadxe2x80x99 uses recycled plastics such as polystyrene, the extrusion process becomes very difficult to implement because of the poor physical properties of the extruded recycled material. It is, therefore, another object of the present invention to provide a process and apparatus to allow extrusion and coating of materials such as recycled polystyrene which have poor physical properties.
Accordingly, the present invention provides apparatus for manufacturing pencils comprising a core material extruder, a sheath material extruder, a crosshead and die assembly for coating the sheath material onto the core material, and a means for sizing the extrudate from the crosshead die, the sizing means is separated from the face of the crosshead die by a predetermined distance to provide a cooling zone in which the extrudate is allowed to cool at ambient temperature, the apparatus comprises means for maintaining the length of the extrudate in said cooling zone between a minimum and a maximum length and sensors for detecting when the length of the extrudate reaches either said minimum or maximum length. The apparatus of the invention allows the manufacture of pencils using synthetic materials even when these are unconventional materials or materials having less than optimum properties, for instance recycled materials. In the cooling zone the sheath material is allowed to expand and the adhesion between the core and sheath are thereby improved.
The apparatus may further comprise a cooling unit between said core extruder and said crosshead and die assembly, the cooling unit being separated from the core material extruder by a predetermined distance providing a second cooling zone in which the extrudate from the core material extruder is allowed to cool at ambient temperature.
The invention further provides a method for extruding pencils having a core and a sheath, comprising loading the core and sheath materials into respective extruders, cooling the core material and coating the sheath material onto the core material in a crosshead and die assembly, cooling the extrudate from the crosshead and die assembly and sizing the extrudate to a predetermined diameter in a sizing means, the step of cooling comprises allowing the extrudate to cool at ambient temperature and maintaining the length of extrudate in a cooling zone between an outlet of said crosshead and die assembly and said sizing means between minimum and maximum lengths. In the cooling zone the sheath material is allowed to expand and the adhesion between the core and sheath are thereby improved.
The extruded sheath material preferably comprises an endothermic blowing or foaming agent.
The core material may be initially allowed to cool at ambient temperature before cooling in a cooling unit to provide a rigid and straight core material to be fed into the crosshead and die assembly.